Microwave ovens use the principle of the excitation of molecules to prompt the heating and cooking of food. To this end, the food is placed within a cavity of the oven forming a Faraday cage in which radiation of waves at very high frequency is produced by a transducer called a magnetron. A part of the electrical power consumed by the magnetron and by its supplying device is transferred to the food in the form of heat.
The energy balance of this reaction is complex, accordingly, it will simply be noted that, for a given value of supplied power of the magnetron, the time needed to bring food to a given temperature is a function of various parameters. Hereinafter, an operating cycle of the oven to bring food to its ideal temperature of consumption is called a "heating cycle". The duration of a heating cycle depends, for example, on the nature of the food (liquid, meat, vegetables, etc.), the volume of the sample to be heated, its density, its initial temperature, etc.
In the prior art (FIG. 1), the user programs the duration of a heating cycle by a timer or similar programming device that is accessible on the control panel 2 of the oven 1. This is done after placing the food to be heated inside the oven and closing its door 3. To carry out this programming operation, the user must estimate the appropriate duration of the heating cycle on the basis of an assessment of the above-mentioned parameters and of his own experience in the use of the oven.
This estimate is, by its nature, highly approximative. This is why it is not unusual for the duration of the heating cycle programmed by the user to be actually far too short or far too long. In the former case, a new heating cycle has to be programmed. In the latter case, there may be phenomena of boiling in the food that get splashed about and thus ruined. The user is therefore often forced to look through the window of the oven door to see how the appearance of the food is progressing in the course of the heating cycle so that, if necessary, he can stop the cycle before it ends.
In certain cases, the control panel 2 of the oven 1 furthermore has keys or pushbuttons by which the user can program different values of the operating power of the oven, namely the values of the supplied power of the magnetron. The user must also make a choice, for example, between a key corresponding to non-frozen solid food, a key corresponding to liquids, and a key corresponding to frozen food ("defrost" or "thaw" key), identified by appropriate symbols. These three keys correspond, for example, to instantaneous values of the supplied power of the magnetron. These values may be equal, respectively, to 300 watts, 500 watts and 1000 watts.
However, far from eliminating the above-mentioned problems entailed by having the user program the duration of the heating cycle in an approximate fashion, this additional user choice has a consequence of introducing an additional degree of arbitrariness into the programming. Accordingly, there is an even greater risk of an unsuitable duration being programmed.
Furthermore, the plurality of programming parameters (operating power and duration of the heating cycle) give a more random character to the interpretation and storage of the results of successive programming operations. The user therefore has greater difficulty in using his experience of the oven.